Yesterday I went into the dojo early to take ukemi for a sempai. I just did tanto attacks, whichever ones he specified, starting quite slowly, then building up speed, then slowing down if he wanted to concentrate on something...I never felt like I "tanked" for him, nor did I feel like I mindlessly resisted. I tried to make my body a mirror to show him what it was he was doing, if that makes any sense. The process of doing so made me really focus on feeling what it was he was doing, and reflecting it back. Since I didn't know what technique he would do any time, I couldn't prepare the way we so often seem to do (moving towards a roll we know is coming). I never actively resisted or countered, but if I could really easily turn away or keep the knife for reasons other than the slow pace we agreed on, I did, so he had a chance to correct himself or retry from there.
The whole thing was incredibly fun and incredibly instructive.
I agree that, for me, the most valuable thing is, are we learning? or more meaningfully, Are we learning what it is we want to learn?